The disclosure herein relates in general to electronic circuitry, and in particular to a method and circuitry for inhibiting or preventing delamination at the interface of the die attach/mold compound and the die pad of a semiconductor device.
A leadframe-based semiconductor device package is a commonly used integrated circuit (IC) package. The leadframe typically includes a chip mount pad (also referred to as a die paddle) for attaching the IC die or chip to the leadframe, and a plurality of lead fingers or conductive segments which provide a conductive path between the chip and external circuits. A gap between the inner end of the lead fingers and the chip is typically bridged with bond wires attached to bond pads on the chip and to the inner end of the lead fingers. Bond wires can be formed from various materials, such as gold, copper, aluminum, or an alloy thereof. The outer ends of the lead fingers remote from the IC chip can be electrically and mechanically connected to external circuitry. After assembly of the chip to the leadframe, the chip, the bond wires, and a portion of the leadframe can be encapsulated in mold compound. Non-limiting examples of leadframe-based semiconductor devices are disclosed in U.S. Pat. Nos. 7,045,456; 7,268,073; 8,133,761; 8,304,871; and U.S. Publication Nos. 2007/0273010; 2010/0193944; 2013/0075890; 2013/02778816; 2013/0277825; and 2014/0191378, which are incorporated herein by reference.
As the encapsulated semiconductor device undergoes temperature cycling during device testing or use, it is known that thermomechanical stresses are induced at the joints or interfaces between dissimilar materials used in the fabrication of the device. The stresses are primarily induced due to differences between the coefficients of thermal expansion (CTE) of the various materials. For example, metal used to fabricate the leadframe assembly expands or contracts differently than plastic resin material used as the mold compound, thereby causing delamination of the mold compound from the leadframe. These stresses, which may be repeatedly induced during hundreds or thousands of temperature cycles, tend to fatigue the joints and the interfaces and can result in separation between the leadframe and the mold compound. If delamination occurs, the device can fail as a result of physical damage during separation or from corrosion resulting from the intrusion of moisture or an impurity into the delaminated package.
Die pad delamination can also occur during high temperature storage (HTS) of the semiconductor device. During HTS, the semiconductor device can be exposed to temperatures equal to or greater than 200° C. for extended periods of time. It has been observed that long-term storage of semiconductor devices at >=200° C. for >2000 hours can readily lead to die pad delamination. The delamination is partly or fully due to the degradation of the interfacial interaction at the interface of the die attach/mold compound and the leadframe.
Various strategies have been used to address the delamination problems. One strategy includes the use of adhesion promoters in die attach or mold compound formulations. The adhesion promoter is used to increase the strength of adhesion between the interface of die attach/mold compound and leadframe. However, increased amounts of adhesion promoters in die attach material increase material cost and do not always result in improved adhesion. Also, increased use of adhesion promoters can adversely affect the die attach properties, such as the formation of voids during curing, thus decreasing, rather than increasing, the strength of adhesion between the interface of the die attach and the leadframe. Another past practice to address the delamination problem was to roughen the leadframe using chemical or mechanical processes. The roughening of the leadframe will increase the surface area of adhesion, thus improving adhesion and decreasing the incidence of delamination. In some, but not all, applications, the roughening of the leadframe would improve delamination; however, the additional processing step of roughening the leadframe added to the significant manufacturing costs of the semiconductor device. Roughening of the leadframe can also result in bleeding problems which can adversely affect the properties of the semiconductor device. Another past practice to address the delamination problem was to plasma clean the leadframe prior to attaching the die to the leadframe. The plasma cleaning of the surface of the leadframe, while not as effective as roughening the surface of the leadframe, could be used to promote adhesion between die attach/mold compound to the leadframe. As such, in some, but not all, applications, the plasma cleaning of the leadframe could improve adhesion.
In view of the current state of the art, there is a continued need to inhibit or prevent delamination at the interface of the die attach/mold compound and the die pad of a semiconductor device without significantly increasing the complexity and/or costs of forming the semiconductor device.